Natural and synthetic polymers are rarely used (especially in the field of textiles) in their original color. The coloring of the polymers is carried out not only for the sake of their appearance but also for other purposes, for example, to improve the stability of the polymers to light by incorporating TiO.sub.2, optionally mixed with other light protective agents and other substances.
Polymers, for example in the form of fibers or textiles, employed in the textile industry may be colored by various methods. Dyeing and printing processes which employ soluble dyes are known, e.g. direct dyes for cellulose fibers, acid dyes for wool, basic dyes for polyacrylonitrile, and the like. Another method consists of dyeing and printing using pigments which are applied to the surface of polyester fibers, for example, in the form of dispersions, and which diffuse into the fibers and dissolve in them. Direct incorporation of dyes in the fibers by spinning is also known. Dyeing and printing textiles can also be carried out by pigment dyeing and pigment printing processes using binders. In these cases, the binder binds the pigment to the surface of the fiber or textile.
For practical reasons, the number of processes commonly employed for coloring polymers which are used as coating agents is much smaller than those employed for coloring and printing the textiles themselves.
For coloring coating compounds for textiles it is most common to use color pigments which are generally insoluble in the polymers. Thus, for example, in polyurethane coating compounds it is preferred to use color pigments of either organic or inorganic origin. The color yield depends on the particle size of the color pigments. The larger the surface area of the pigments, that is to say the more finely divided the pigment, the greater is the color yield.
Since it is extremely difficult to obtain pigments of reproducible particle size by mechanical methods using bar looms and therefore more difficult to form patterns, it has become customary in some cases to work up the pigments in large batches and to control the particle size by coating the small particles of pigment with a coating resin. This prevents the finely divided pigments from reagglomerating. The disadvantage of this method is that when preparing the pigment paste the coating resin must always be compatible with the polymer with which the pigment is to be covered.
The use of purely inorganic or organic color pigments has also some other disadvantages. Inorganic pigments often have a rather low intensity color and the range of shades is relatively restricted. Organic pigments provide a wide choice of intensive and brillant shades, but they tend to migrate under the action of heat and sometimes even to sublimate. This tendency is increased by, among other things, finishing the surface with silicons.
The dyeability of polyurethane coating compounds also depends on the system. For one and two component polyurethanes, the solid substance which is applied as a solution in an organic solvent can be colored both with finished (coated) and unfinished pigments (unfinished pigments must be ground). Only in some cases can aqueous polyurethane suspensions be dyed satisfactorily with pigment preparations. Moreover, polyurethane coating powders are not colored to a sufficiently high intensity with pigment preparations.
One major disadvantage of one-component and two-component polyurethanes which contain color producing groups which are chemically incorporated in the polymer molecule is that they cannot be blended indiscriminately with any uncolored or differently colored polyurethane solutions because polyurethanes which differ from each other chemically are frequently incompatible with each other, depending on whether they have been synthesized from polyesters or polyethers, aliphatic or aromatic polyisocyanates or chain lengthening agents.